A family of guanine nucleotide binding proteins, G proteins, function as receptor effector couplers. Two distinct G proteins, Gs and Gi, couple stimulatory and inhibitory hormone receptors, respectively, to adenylate cyclase. A homologous retinal protein, transducin, couples a light receptor, rhodopsin, to cGMP phosphodiesterase. Go is a G protein of unknown function recently discovered in brain. We have purified G proteins, raised antibodies to them, and use these to study the structure, function, and distribution of specific G proteins. These studies provide important insights into the mechanism of signal transduction across the cell membrane.